The present invention relates to the general field of turbine engine blades made of composite material and having a root in the form of a bulb for mounting on a rotor disk via a dovetail type connection.
The intended field is that of gas turbine blades for aeroengines or for industrial turbines.
Proposals have already been made to make turbine engine blades out of composite material. By way of example, reference may be made to International patent application number WO 2010/061140 filed jointly in the names of Snecma and Snecma Propulsion Solide, which describes fabricating a turbine engine blade by making a fiber preform by three-dimensional weaving and by densifying the preform with a matrix.
Furthermore, for mounting such a blade on a rotor disk, it is known to give the blade root the shape of a bulb. The blade root with its bulb shape co-operates with a slot of complementary shape formed in the periphery of the rotor disk in order to retain the blade radially on the disk by a dovetail type connection.
With a blade made of composite material, the bulb shape of the blade root is generally made during weaving of the fiber blank that is that constitute the blade by forming extra thickness in the blade root, this extra thickness subsequently being machined to the final shape of the bulb. In practice, the extra thickness is usually obtained by adding an insert during the weaving of the fiber blank.
Nevertheless, such a method of fabricating a composite material blade with a bulb-shaped root presents numerous drawbacks. Specifically, making the insert and putting it into position during weaving of the fiber blank for the blade constitute operations that are very difficult. Also, that method requires the extra thickness of the blade root to be machined to its final shape, thereby having the consequence of spoiling the intrinsic properties of the composite material, in particular by cutting fibers in the bearing surfaces of the blade root. This causes degradation of the attachment of the blade in terms of its mechanical strength.
Also known, from document FR 2 941 487, is a solution for mounting a blade made of composite material on a rotor disk, in which the blade root is clamped between metal plates that are fastened by means of a welded peg. With that solution, the main force retaining the blade on the rotor disk is taken up by shear in the peg and by compression against the hole in the composite material. Nevertheless, the difference in expansion between the metal of the plates and the composite material of the root either gives rise to thermal shear stresses if the fastening is rigid, or else to uncertainty about the positioning of the bearing surfaces if the fastening is made with slack.